1. Field of the Invention
The present invention relates to an automatic vehicle cruise control system and in particular to an automatic vehicle cruise control system in which a throttle valve opening rate is controlled for maintaining the vehicle-speed at a constant value.
2. Description of the Prior Art
A conventional automatic vehicle cruise control system of this type is disclosed in Japanese Patent Publication No. 60-58046 issued on Dec. 18, 1985. In this system, an electromagnetically operated clutch means is interposed between a throttle valve and a motor for driving the throttle valve. When the clutch means malfunctions due to an unexpected problem, a control circuit will operate the motor so as to decrease the opening rate of the throttle valve.
However, if a driver turns off the main switch of the automatic vehicle cruise control system immediately upon recognition of the trouble therein, the opening of the throttle valve to be reduced is fixed as it is so that the vehicle-speed cannot be lowered.